Adjustable attenuator



Febr-9, 1960 H. F. ENGr-:LMANN ETAI- 2,924,793

ADJUSTABLE ATTENUATOR Filed March 9, 1956 2 Sheets-Sheet 1 H. F. ENGELMANN ETI'U- Feb. 9, 1960 ADJUSTABLE ATTENUATOR Filed Match 9, 1956 2 Sheets-Sheet 2 INVENTOR S HERBERTWGEZMGA/N BY MAI/RICE ARQ/T ATTORNEY United Stareslm zgszj4g79s' -ADJUSTABLE ATr-ENUATOR Herbert Ff Engelmann, MountainmLkes, alidMau'rioe Arditi, Clifton, NJ., lassignors to InternationlTelephone and TelegraphCorporatio'n, Nutley`,. N.J.`,"a corporation of' Maryland applicati@ March v9, .1956,l-serial No.. 570,448

' 10-Claims. I'(Cl.'13'33'8 1) `yThis invention relates tofradi'o" frequenoyfattenuators vrand more partic'ularlyto'anattenuator'capable 'of' uniform impedance "matfc'h adjustment. over -a"broad band of "fre- ""quejncies andfor diterent attenuationdjstme'nt.

"Itlhas lreeertlybeen iiiseovered"that `agsiinple lineapplied to fa layeroffdieleetric :materialtby known printed circuit tethni'ques, `one being Vinf the 'ifo'rrnv ofl a planar 'conductor "aridthe 'other `vz 1""riairowillat'i stripllikebline 'conductor'. "The'pr'opa'gation of ra'dio frequency energy is in a manner similar to the TEMV mode "in" "that! the distribution fof the electric" field is substantially Hthe same asfthat "Which"`occurs` between 'jone'conductor an'd the neutral fplane of `a" truly.' paralleljf'two-wire` transmission line'. M' R j Certain ttypesoi microstrip `attenuators,J btlrxed and' piv'oted, rarecliselosedfintlrefpatentof "D Grieg; "Engelmann andil. ""AjKosti-izafNo. 227252535, yand i the application 'of H."En'g'el'mann, Serial No. `4l`3,`805, 'flleClll-Vlarch3,1954.` While 'these d'iselosresare eieetiVe as attenuators` forf the microstrip line; wehave'l foundthat fy'it "is-essential' for uniform irnpedance'imath over broad -band of "frequencies and .for 'differentv 'attenuation ad- Ij"u`st`rnc'ent lto provide some means toniaintain :the at- *t'enuatorf pad 'truly planar with"` respect toJ 'the a microstrip v conductorsthroughout-all positions f adjustment. This v isfnecessarysincethe Lpaid tendsftoib'en'd -andcause the `edge f-'theref ftofdip; into theplaleof thestripondu'ctor when it'- 'extendsftbeyond 'the laterali dgefthestripconductora This dipping of the pad upsetstheimpedanceinatching -Vfrom'one-adjustrnent tofandther. `"It isVv one yof the objectsI ofthe' present"inventiom'ftheref fore, Itof provide 'a wide: ibaiid'f'r'elativ'ely simple, sturdy,

'f' compact *and -i easily l'constructed adjustable attenuator 'having uniform impdance' matchingcharacteristics -with 'lrespeet l 'to 5 fallA ofitseaidjustment' positions relative' to Jthe "conductors *of f the"mit:`rostri1vg typeof I waveguide.' 'Another object' io'f the"inve'n'tioriis'V to provide an attennator tially free" offlrriicrojihonicsdue 'to' vibrations. i Stilll another objectis"topro'vide af novel means for 'adjusting'the position 'of lan* attenu'ator' `card or' f'pad relative the strip "conductors,

1 Onefof 'thef features f\ the inventionv isf'the yprovision,

rinfconjunction 'with anattenuat'or pdfdisposed'for' vlateral sliding zzmovementf relative? to' the-#strip -:conductor, of a 4 plate ofr :dielectrictmaterial interposed betweenthe istrip conductors f and y, the -S attenuator;..pd. rlhis.; -plater iis -:Lmain- Vtained .ytrulyeplanarx.andgparallel ltostheM conductorstofathe microstripwaveguide 1. Means1are:provided-togadjnstethe 2,924,793 a Patented Feb.`49, 1950 Yice lposition ofJthe padcrosswise of thisplateftheplateop `erating as a base which supports the padini altruly parallel. relationship with respect .to 'the planes of Ithetwo conductors. Pressure means, .including the i adjustment controlfreders thepad free -of microphonics which are .normally due to vibration.

The above-mentioned'and otherlfeatures land bjectsof `this invention land the manner f atta.ini ng lthem will become more apparent by reference to the lfizvllovs'i'ng'descripltiontakenin conjunction with the accompanying-'drawings, wherein: f j

' Fig.'1 is a cross-sectional -view of `ainicrost'rip line. and an attenuatoripad'thereon; l

'."Fig,A 2 is a cross=s`ectiona1lview of a microstr-ip` line 'with 'an a'tt'enuatoi-padfdisposed in a trulyplanar relationship in accordance with'the present invetion;`

'Fig.`3 is a view`in` plan of an attenuator utilizingfthe mierostrip. principle and attenuator pad arrangementillustratediniFigfZforuseiin-a coaxialline; v

Fig. 'f4'. is a cross-sectional .view Itaken .along fline ofiFigfS; .j

Y Figs. 5 and' 6 are cross-sectional views taken 'long'liue "f5-."'5iand`6 16,4 respectively,` of Fig. 4;l U

.Fig"7v is a, plan view of a modified form of an Vattenuator unitaccording tolthe present invention; and A Figf`8 is a view partly in 'section taken along line 8-8 ofFig. l7. y

Referring`.to"Figs."l axid"2,V a cross-sectional view ot'. a 'microstrip waveguide is shown to include -a'rst orplanar conductor :1, a second or strip conductorZ an'da ,layer of dielectric `material 3` which separates the-.conductors 1'an'd12 inparallelrelation. "The-.dielectric material may .comprise polyt'ethyl'ene,l polystyrene, "Teon, ber glass `pr" lam'inations of'tbery glass and Teon,-or othersuit- 'fable material of 'dielectric quality, or iftmicrostrip struc- 'ture'permits the dielectric may even be air.r "The `conductor 'strip`2co`mlprises a tial: conductive stripvof vdefinite thickness which is attached tothe upper sufaceof theldielee- "tric: layer 3. iThe strip, "for example, may'beappliedby 'anyfthe known printed circuit techniques .suchassilk -screening,felectrofpl-ating Vand/or etching. AI-"ig'l shows 'an' attenuator pa'd 4 resting-"directly -npon 'thefstrip 2 with `thejportions -of the pad extending beyond the lateral edgesof"the:strip 2 bending. into'the `plane of the'strip. Since the :electric eldj exists between the opposed. sur- Efaces o`f `the`conductors `1 and'land includes afring'effield betweenthe dgesof the strip 2 and the topportionfthere- "'fadjae'ent the edges anydeviationof thepad- 4' fromla planarcondition upsets the impedance match intadjust- 'ment .from ione fposition to .another of thelspad.` laterally "withrespect to"the strip 2.V InLEigQZ theJ-p'ad 4` `is.shown 'to besupportedby afdieletric plate' Srpreferblyof lossl'lesseharaeteristiclwhich. is maintained truly planar with respeettoftheplanesfofthe conductors 2 and 1. "The portion of plate '5; overlying the conductor "2 located in the rfringe feld is Y preferably tapered symmetrically "(5a,A Fig. 7) tofprovide f ormpedancev match. throughout Ithe frequency 'rax: lge. Wherelthefdielectric constant .of the plate is Ycloseto "unity this' taper could be dispensed with. While the.. plate 5 may extend,considerably.beyorid theedges fthestripZv and be permittedI to restonthedi- `electric;ifpartieularly-where 'the conductor21`.s..exeep tionlly "thin, it "is best to,v provide spacers f6 ,.to tinsure lthe planarrelationship in the area of "thepad" 4 and its =zone f'adjustment. "Referring`5to' Figs.' 3 wtoJ @an-'embodiment ofthe nvenf'tioni--is shown for use"'with'fcoaxil lines.. The uiiit'ncludes Va long rectangular housing r7 `having side "walls 128fa`n'd`9, endfwlls1`0and y11,"-a`1d"a bottom Wall J12. L'Iheu-bottomiwall isvjprovidedewith two"ope`nings '13 aridl'14 aadjacentlathefierids'ifofitheffhousingthroughewhichzfcoaxl -:f'coupling::is made. A section:f-vmierostriplsidisposed Within-the housing and is identified by the reference characters 1, 2 and 3. The ground conductor 1 is supported by the spacer and the coaxial coupling connectors 16 `and 17.. The outer lconductor 18 of the coline isl coupled to the bottom wall 12 for extenslon through the opening therein and in electrical connection with the' ground conductorl; The inner conductor 19 2` is the plate 'A5, the outer edges of which are supported by spacers 2 0; The attenuator pad 21 of resistive mal. ferial is disposed for sliding movement on the dielectric plate 5. One edge 22 of the pad 21 is angled from the center thereof to provide abroad angled taper. This edge22 constitutes the vforward edge of the operatlng portion o f'the pad and controls the change in attenuation and its symmetrically tapered shape avoids change in impedance match as it is adjusted laterally with respect to the strip 2.

The end'walls 10 and 11 are provided with bearings 23 and 24 which support a shaft 25. The shaft 25 car- 'ries four resilient rollers 26, 27, 28 and 29, which engage the surface of the pad 21 for friction drive. The pad 21 is'provided with a guiding ridge 30 which is receivable in an annular channel 31 formed in the shaft 25. This ridge 'guide maintains the pad in proper alignment during adjustment. The rollers lare driven by a knob 32 for adjusting the attenuator pad. In order to minimize effects of vibration two spring elements 32 and 33 are provided, each being in angular form with one end secured to the'side wall 9 and the other end provided with a rounded contacting surface 34 adapted to engage the upper surface of the attenuator pad 21V closely adjacent the axis of the shaft 25 and preferably directly between the shaft and the pad 5. The shaft 25 is providedwith annular grooves 35 and 36 to accommodate the spring elements 32 and 33. These spring elements together with the resilienceuof the roller maintain the attenuator free of microphonics due to vibrations.

` j Figs. 7 and 8 show an attenuatorwhich is particularly adapted for microstrip circuitry. In this embodiment the microstrip is shown to comprise a rst or ground conductor 41, a strip conductor 42 and an interposed layer of dielectric 43. The housing is in the form of a rec tangular frame comprising side Walls 44 and 45 and end walls 46 and 47. The walls'are secured through the -dielectricA layer 43 to the'ground conductor 41 by rivets'or y other fastening means indicated at 48. Disposed in overindicated at 56 and 57. The shaftA is driven by a centrally located worm and pinion gearing 58, 59, the worm gear 58 'being carried by a shaft 60 mounted on the Awalls 44 and 45 and controlled'by a knob 61. The gear 59 is keyed to the shaft 51.

From the foregoing it will be clear that the attenuator padmay be drivenvby a shaft extending through either the ends or the Sides of the housing. While other means for moving vthe pad may be arranged, the present shaft and roller arrangement is preferred since it-urges with resilient force the pad against the supporting plate. This resilient pressure is supplemented by the spring elements 32 and 33 which are also employed in each embodiment plate. The attenuator herein disclosed has been usedv for 4. the impedance match thereof. The attenuation was found to be substantially linear throughout the range of attenuation adjustment, and as an example, a pad of about 4 inches long provided an attenuation range from about 1 to 12 decibels for a frequency of 3,000 rnegacycles, and the maximum attenuation provided by this pad varied from 2 db to 40 db in vtherange of frequencies from 400 to 10,000 megacycles.' In this range the impedance match gave an input voltage standing wave ratio below 1.3.

While Wehave described above the principles of our invention in connection with specific structures, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as Yset forth in the `objects thereof and in the accompanying claims.

We claim:

1. An adjustable microwave attenuator of substantially i i uniform 'impedance matching characteristics, comprlsing first and second conductors, means disposing said conductors inclose dielectrically spaced parallel relation, said first conductor being wider Ithan said second conductor to present thereto a planar conducting surface whereby the main electromagnetic field is distributed between the opposed surfaces of said conductors with a fringe field extending from the side edges of said second conductor and the top portions there-adjacent toward said first conductor, a plate of dielectric material disposed in parallel overlyingy relation to =said first and second conductors and in said fringe field, a pad of resistive material disposed on said` plate, said pad having an operating portion of symmetrical uniformly tapered shape andmeans to adjust the position of the operating portion of said pad transversely with respect to Isaid second conductor and said fringe field.

2. An adjustable microwave attenuator of substantialvly uniform impedance matching characteristics, comprising first `and second conductors, means disposing said conductors in close dielectrically spaced parallel relation, said first conductor being wider than said second conductor'to present thereto a planar conducting surface whereby the'main electromagnetic field is `distributed between the opposed surfaces of said conductors with a fringe field extending from the side edges of said second conductor and the top portions there-adjacent toward said first conductor, v a plate of dielectric material disposed in parallel overlying relation to said first and second conductors and in said fringe field, a pad of resistive materialdisposed on saiddplate, said pad having an operating portion of symmetrical uniformly tapered shape, means to' adjust thel position of said pad transversely with respect to said second conductor, and means resiliently urging said pad against said plate to minimize the microphonic effects due to vibration.

3. An adjustable microwave attenuator of substantially uniform impedance matching characteristics, comprising first and second conductors, means disposing said conductors in close dielectricallyspaced parallel relation,

. `whereby the main electromagnetic field is distributed between the opposed surfaces of said conductors with a varying the attenuation for -frequencies between 400 and fringe field extending fromvthe side edges of said second conductor ,and the top portions there-adjacent toward said first conductor, a plate of dielectric material disposed in overlying relation to said first and second conductors and in a plane substantially parallel to the said planar surface and in said fringe field, said plate having the edges thereof whichcross said second'conductor ltapered symmetrically with respect to said second conductor, a pad if resistive material disposed on said plate, said pad having a forward edge'tapering at an angle to the center line of said second conductor, means to adjust the position of said padlaterally with respect to said second --conducton and means to maintain said pad in '1'y1 unitari-nf assed-sns align4M ment witty:si-iinett'nisvrssly' ofsaideenter line-'dui-L ingiadjnstiiientinsvjeaisnt thereseV /tsAnasdjnstabief ciewavefiattenuater offsubstantisiingsfrlst siidiseentrfeennncttiistmeans disposing 'said'lesnductors in said dielectrically spaced parallel relatio saidl first conductor being, wider thanysaid secondr conductor Vfringe field extending from the side edges of said second conductor and the top portions there-adjacent toward said first conductor, a plate of dielectric material disposed in overlying relation to said first and second conductors and in a plane substantially parallel to the said planar surface and in said fringe field, a pad of resistive material having a symmetrically tapered operating portion Vdisposed on said plate, a frame, a roller supported by said frame in engagement with said pad and means for rotating said roller to position said pad transversely with respect to said conductor and said fringe field.

6. An adjustable microwave attenuator of substantially uniform impedance matching characteristics, comprising a housing, a microstrip waveguide section in said housing including first and second conductors and a layer of dielectric material disposing said conductors in spaced parallel relation, said first conductor being wider than said second conductor to present thereto a planar conducting surface whereby the main electromagnetic field is distributed between the opposed surfaces of the conductors with a fringe field extending from the side edges of said second conductor and the top portion there-adjacent toward said first conductor, a plate of dielectric material disposed in overlying relation `to said first and second conductors-aud in a plane substantially parallel to said planar surface, a pad of resistive material having a symmetrically tapered operating portion disposed on said plate, said housing having bearing means, a roller supported by said bearing means in engagement with said pad, and means extending outside said housing for rotating said roller to adjust the position of said pad laterally with respect to said second conductor.

7. An adjustable microwaveattenuator of substantially uniform impedance matching characteristics, comprising a housing, a microstrip waveguide section in said housing including first and'second conductors and a layer of dielectric material disposing said conductors in spaced parallel relation, said first conductor being wider than said second conductor to present thereto a planar conducting surface whereby the main electromagnetic field is distributed between the opposed surfaces of the conl ductors with a fringe field extending from the side edges of said second conductor and the top portion there-adjacent toward said first conductor, a plate of dielectric 'ateniig' characteristics; certipris-v ai symmetrically tapered-operating:pertien `dispesed1 on' means, arollerl saidAll plate, `said hosiiigihalving bea" ngagement with supported4 byA said: hearing means-i tion oftheoperatingfper'tionfoff said-:paar relative t's'aidsecond-'- eonductor,- sidl roller? havinga` circular groove therein;` said`p`ad having airitge'freceivable insaid groove to' maintain 'said pad? in lprtnaerv alignmentiwithrsaidroller-1 and spring elements@carried/byE said! housingfhaving por# tions adapted to engagei andi press Lsaid lpadl against said plate of dielectric.-

8;' Ani adjustabley microwave" attenuator ofasubstantiallyf uniform i impedance? matching icha.racteri`sties,'- cornprising' aimicrostrip'fwaveguide*sectioniincluding firsti and second eonductorsfand,t arlayer of dielectric material disposingsaid conductors 2in1 spaced parallel relation, saidfirst y conductor fbeingswider i than saidfsecoiidlconductof to present thereto azplan'asi conductingsurfce wherebythe main electromagnetic field is distributed between the opposed surfaces of the conductors with a fringe field extending from the side edges of said second conductor and the top portion there-adjacent toward said rst conductor, a plate of dielectric material disposed in overlying relation to said first and second conductors and in a plane substantially parallel to said planar surface, a pad of resistive material having a symmetrically tapered operating portion disposed 'on saidplate, means toadjust the position of the operating portion of said pad laterally with respect to said second conductor, means for coupling a coaxial line to each end of said section, said last named means including means for coupling electrically the outer conductor of said coaxial line to said first conductor, said first conductor having an openingV therethrough coaxially with respect to said outer conductor and means coupling the inner conductor of said coaxial line through said opening to said second conductor.

9. An adjustable microwave attenuator of substantially uniform impedance matching characteristics, comprising a rectangular housing, a microstrip waveguide section disposed lengthwise in said housing including first and second conductors and a layer of dielectric material disposing said conductors in spaced parallel relation, said first conductor being wider than said second conductor to present thereto a planar conducting surface whereby the main electromagnetic field is distributed between the opposed surfaces of the conductors with a fringe field extending from the side edges of said second conductor and the top portion there-adjacent toward said first conductor, a plate of dielectric material disposed in overlying relation to said first and second conductors and in a plane substantially parallel to said planar surface, a pad of resistive material disposed on said plate, said pad having a symmetrically tapered operating portion, the end walls of said housing having bearing means, a roller supported by said bearing means in engagement with said pad, means to rotate said roller to adjust the position of the operating portion of said pad laterally with respect to said second conductor, the bottom wall of said housing having two openings one adjacent each end, means for coupling a coaxial line through each of said openings, said last named means including means for coupling electrically the outer conductor to said rst conductor, said first conductor having an opening therethrough coaxially with respect to said outer conductor and means coupling the inner conductor of said coaxial line through said last named opening to said second` conductor.

10. An adjustable microwave attenuator of substantially uniform impedance matching characteristics, comprising a housing, a microstrip waveguide section disposed in said housing including first and second conductors and a layer of dielectric material disposing said conductors in spaced parallel relation, said first conductor being wider than said second conductor to present thereto a planarconducting surface whereby the main electromagnetieheld is distributed between the opposed sur.

f acesv of the conductors witha fringe eld extending from the side edgesj of, said second conductor and the top portion there-.adjacent toward said first conductor, a plate of-dielectric material disposed in parallel overlying relation to said rst and second conductors,.and in said fringe'eld, a-pad of` resistive material having a symmetrically .tapered operating portion disposed on said plate, said housing having bearingmeans, a roller supported by said bearing means inuengagement with said pad, means to rotate said roller to adjust the position of, said pad laterallywith'respectrto-sad second conductor, spring elements carried by said housing. having portions adapted, to engagefand press; said pad; against said plate of dielectric, -the, bottomwall of,said'housing having` two openings spaced a part,n1eans4 for coupling a coaxialline through each of vsaid openings, said last named means including-means for coupling electrically the outer eonductorof said vcoaxial linejto said iirst conductor, said rs't conductor having an opening Atherethrough. coaxially with respectztoisaidg outer conductor and means couplingthe inner conductor. of saidl vcoaxial line throughsaidf, last .named openingto` said second Conductor.. ,Y

References Citedin the file of this patent 'ff v f UNITED srTEs'fPATENrs' 1953, page 7 relied on.

Grieg et a1. t.- \Nov. 29, 1955- 

